Advanced CMOS nodes require multi-gate architectures in order to achieve sufficient electrostatic control to modulate current in short channels. A multi-level nanosheet device is a promising architecture for advanced nodes as it may offer excellent mobility, electrostatic control and possible layout enhancements. In addition, nanosheet architecture may compare favorably to the alternatives because the roughness of the channel/dielectric interface is limited by the precision of broad area epitaxial growth and selective etching instead of, e.g., lithography and anisotropic etching as is the case for FinFETs (field effect transistors)
Adoption of nanosheets in advanced CMOS nodes (i.e. <=7 nm) face the following several challenges. One challenge is that highly selective etches must exist in order to ensure that the thickness of the nanosheet remains uniform from middle to end. Prior techniques use Si/SiGe superlattices where either Si or SiGe layers comprise the active material in the final structure, whereas the SiGe or Si layers, respectively, serve as sacrificial layers to be selectively etched away. The selectivity of etches for the Si/SiGe system is limited to relatively low values due to the chemical similarity between Si and SiGe. However, this problem has been addressed by U.S. patent application Ser. No. 14/830,622 filed on Aug. 19, 2015, “Rectangular Nanosheet Fabrication Method”.
Another challenge facing adoption of nanosheets in advanced CMOS nodes is the introduction of strain into the channel (nanosheet channel layers) of the final structure. In order to meet advanced CMOS performance targets, it will likely be necessary for significant strain to be engineered into the channel for n- or p-type Si, SiGe or Ge channels and even some p-type III/V channels (e.g., GaSb). Prior techniques utilize a method to fabricate nanosheets whereby any strain introduced by the original superlattice is lost during processing.
What is needed is a fabrication method that will result in a nanosheet final structure that retains biaxial strain from the original superlattice.